This invention relates to indicating devices in general, and in particular to devices for indicating the proper adjustment of a frequency-compensated attenuation probe to provide an impedance match to the input circuitry of an electronic instrument to which a probe is coupled.
Attenuation probes have long been used in connection with oscilloscopes to permit a signal under test to be divided down to an amplitude usable by the input circuit of such oscilloscope. Because oscilloscopes are general purpose instruments, they have wide-band capabilities to permit the instrument to respond to a broad range of frequencies. The input circuits thereof have necessarily been frequency compensated to minimize distortion of an input signal. The attenuation probe includes passive components to provide a precise impedance match between the probe and the input circuit, and such probes typically include one or more variable capacitors to permit an operator to properly compensate the probe.
A typical method of compensating the probe for use with an oscilloscope is to apply a precise square-wave reference signal to the probe while viewing the resultant signal on the oscilloscope screen. Distortions in the square wave produced by the impedance mismatch are indicated on the wave form by either a peaked or rolled-off leading edge of the square wave. The probe may be adjusted while watching the display to adjust the square wave to a square leading corner.
Other electronic instruments having wide-band capability, such as counters and the like, have come into wide usage, and such instruments lend themselves to usage of an attenuation probe therewith to reduce the amplitude of an input signal to a usable level. However, these instruments do not have a display device for viewing an input waveform and it is difficult to properly compensate the probe without the use of an oscilloscope.